I don't think it can import files from any other programs.
There's nothing in the menu's, but I don't know if there are any external conversion programs.

With PBP 2.60A, you can work directly with your PBP code and it will single step through the program as it's simulating. The internal editor is terrible, but you can use MCS as an external editor. It doesn't use MCS to single step, but it's easier to write stuff.

You can also integrate it with MPLAB, and it will single step using the MPLAB IDE and watch variables, have breakpoints etc, but it reduces the screen space for the simulation.

For me, it's easier to compile programs like I always do with MCS, and just run the .HEX file in Proteus like I would on real hardware. No single stepping that way, but I don't need to single step anymore anyhow.

There's a demo version available.
It's doesn't let you create your own schematics, but there are a bunch of samples that you can write your own PBP code for. It's kind of like buying several development boards. You can't change the hardware, but you can write programs to run on them. And they're free.

I've saved so much money since I got it.
I don't buy chips and parts and breadboards and LCD's etc. anymore.
All my breadboards and electronics stuff are still in boxes after the move to Colorado. Just don't need them anymore.